Superconducting magnets presently in use in MRI systems are basically cylindrical with the magnet in the cryostat and having a bore tube external to the cryostat through which the generated magnetic flux runs. The patient is placed in a prone position in the bore tube virtually coaxially with the longitudinal axis of the cylindrical magnet. Thus, the magnetic lines of force run parallel to the longitudinal axis of the patient.
Among the drawbacks of the present cylindrical magnets is that the patient is wholly within the cylinder; i.e., from the time the patient is inserted into the bore tube, the doctor does not have ready access to the patient. The lack of access is a problem where the patient is in critical condition and needs continual or emergency aid from the doctor. The present systems require the patient to be removed from the bore tube of the magnet to enable access by the doctor. The time taken to remove the patient from the bore tube could be critical.
The cylindrical type magnets have an additional shortcoming. The magnetic lines of flux travel largely through air which offers a much higher reluctance than does a magnetizable material such as iron or steel. Consequently, the magnetic field is inherently weakened by the large proportion of the travel of the flux that is through air.
In the past, attempts have been made to provide more iron in the path of the magnetic flux. This has been attempted by utilizing C-frame magnet sections and/or what are known as "H-frame" magnet sections. In the C-frame construction, an iron or steel yoke passing through an electro-magnet carries magnetic flux to two oppositely disposed pole pieces. Flux crossing the air gap between these pole pieces is guided by pole shaping sections or electrical coils to form homogeneous zones suitable for MRI studies.
In the past, C-frame type magnets have had at least two fundamental problems which have inhibited their use in MRI studies.
The problems are:
1) the C-frame magnet is mechanically unbalanced by the magnetic forces applied between the poles. With the high powered magnets used, there is actually some small, but non-negligible movement when the superconducting coil of the magnet is conducting. The magnetic attraction upsets the homogeneity of the flux between the pole pieces; and PA1 2) the stray field performance in the C-type magnets is much worse than the stray field performance using state of the art devices presently available for controlling the stray fields generated by cylindrical high-field magnets.
Another magnet shape design that has been attempted to improve the presently available magnetic resonance systems has been the H-frame magnets. Such magnets provide a double path of magnetic material for the magnetic lines of force. The H-frame magnet has a number of serious problems when used in high field superconducting devices. With the H-frame, the magnet system has to be enlarged to accommodate a complete cryostat. In addition, there is still no access for the doctor. Thus, as far as patient accessibility the H-frame magnets are similar to the cylindrical magnets presently in use. In addition, the H-frame magnet is unbalanced magnetically and thus, there is great difficulty in obtaining homogeneous fields with the H-frame magnet. There have been suggestions by designers who propose to balance the design with a second cryostat and superconducting coils. However, such designs are inherently even larger and more expensive than the regular H-frame magnets.
Accordingly, those skilled in the art are still searching for new and improved magnets for use in MRI systems.